Apolipoprotein B (apo B) circulates in two distinct forms referred to as apo B100 and apo B48. Apo B48 is encoded by same gene as apo B100 and arises as a result of mRNA editing. A single cytidine nucleotide in apo B100 mRNA is deaminated by a zinc-containing enzyme, resulting in a CAA to UAA-stop codon change (Navaratnam N. et al (1995) Cell 81: 187-195). RNA editing of apo B has important consequences in the catabolism of plasma lipoproteins and the ability to generate hybrid lipoproteins (Davidson N. O. (1993) Ann. Med. 25: 539-53).
Hadjiagapiou C. et al (1994, Nucleic Acids Res. 22: 1874-1879) cloned the apo B mRNA editing protein (HEPR) from human small intestine cDNA. HEPR is the catalytic subunit of an enzyme complex which includes, yet to be identified, complementation factors (Teng B. et al (1993) Science 260: 1819). HEPR contains consensus phosphorylation sites as well as conserved histidine and cysteine residues identified as a Zn.sup.2+ binding motif in other cytidine deaminases. Mutational studies indicated that the putative zinc-cytidine coordinating residues His61, Cys93, and Cys96 and the catalytically active residues Glu63 and Pro92 are necessary for both RNA editing and cytidine deaminase activities (MacGinnitie et al (1995) J. Biol. Chem. 270: 14768-14775). His61 is also required for RNA binding activity.
HEPR is expressed in the adult small intestine and to a much lesser extent in the stomach, colon, and testis (Hadjiagapiou et al, supra). The rabbit homolog of HEPR is only expressed in the small and large intestine, while the complementation proteins, essential for RNA editing, were found to exist in a wide range of organs that do not express apo B, suggesting additional RNA editing enzymes with a more widespread role in the generation of RNA and protein diversity (Yamanaka S. et al (1994) J. Biol. Chem. 269: 21725-21734; Hodges P. et al (1992) Trends Biochem. Sci. 17: 77-81).
mRNA Editing Enzymes and Disease
The principle of therapeutic RNA editing was demonstrated by using cell extracts containing an RNA editing enzyme to correct an aberrant stop codon introduced into synthetic RNA encoding dystrophin protein (Woolf T. M. et al (1995) PNAS U.S.A. 92: 8298-8302). Deamination of a nucleotide in a stop codon resulted in translation read-through and a dramatic increase in expression of a downstream gene.
Apo B editing is a mechanism which determines how much apo B48 is synthesized in place of apo B100 in a specific tissue. Apo B100 is the exclusive apolipoprotein of low density lipoproteins, which transport most of the plasma cholesterol in humans. In contrast, apo B48 is directed to chylomicrons, triglyceride-rich lipoproteins that transport dietary lipids and undergo catabolic clearance much faster than particles containing apo B100 (Young S. G. (1990) Circulation 82: 1574-1594). Apo B editing has major physiological and clinical implications. All apo B100 containing lipoproteins are atherogenic, especially when present in high concentrations. Alterations in apo B100 can cause either hypocholesterolemia or hypercholesterolemia (Innerarity T. L. et al (1991) Adv. Exp. Med. Biol. 285: 25-31). Apo B mRNA editing down-regulates the amount of apo B100 production. Somatic gene transfer of rat REPR, the rat homolog of HEPR, into the liver of mice essentially eliminates apo B100 and plasma low-density lipoprotein without affecting anti-atherogenic high density lipoproteins (Teng B. et al (1994) J. Biol. Chem. 269: 29395-29404).
Alpha-galactosidase is a lysosomal enzyme that is deficient in patients with Fabry's disease. After excluding other possibilities, Novo F. J. et al (1995, Nucleic Acids Res. 23: 2636-2640) proposed that RNA editing is responsible for an observed nucleotide conversion in alpha-galactosidase mRNA from Fabry's disease patients. The enzyme responsible for the nucleotide conversion has not been identified.
Among the main characteristics of psoriasis are abnormal keratinocyte proliferation and differentiation. A search for proteins that are implicated in psoriasis yielded a partial sequence for phorbolin I, a protein upregulated in psoriatic keratinocytes (Madsen P. P., unpublished).
Deamination of RNA nucleotides occurs in the brain. RED1, a double stranded RNA adenosine deaminase expressed in brain and peripheral tissue, edits the channel determinant site in glutamate receptor pre-mRNA. This site controls the Ca.sup.2+ permeability of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors. Another double stranded RNA adenosine deaminase, DRADA, edits a different site in the glutamate receptor pre-mRNA (Kim U. et al (1994) J. Biol. Chem. 269: 13480-13489). Glutamate receptor RNA editing is differentially regulated (Nutt S. L. (1994) Neuroreport 5: 1679-1683). Since glutamate receptors are essential for fast excitatory neurotransmission RNA editing may play a critical role in normal brain function and development. Furthermore, dysfunction of RNA editing may have neuropathological consequences and could be related to neurodegenerative diseases (Nutt et al, supra). Evidence suggests that RED1 and DRADA are members of a larger gene family of enzymes that deaminate nuclear transcripts and have distinct but overlapping substrate specificities (Melcher T. et al (1996) Nature 379: 460-464).
By rationally directing RNA editing towards the transcripts of base substitution mutations many genetic diseases could be treated. Additionally, directed RNA editing could be used to treat any disease in which specific changes in gene expression would be therapeutic. A new mRNA editing enzyme could satisfy a need in the art by providing a new means for altering mRNA and affecting gene expression. This could allow new treatment options for psoriasis, atherosclerosis, neurological disorders, and any condition in which a specific change in gene expression would be beneficial.